Image forming method and charged particle beam apparatus

ABSTRACT

An image forming method and a charged particle beam apparatus suitable for suppressing the inclination of charging when scanning a two-dimensional area with a charged particle beam. A third scanning line located between a first scanning line and a second scanning line is scanned. After the first, second and third scanning lines have been scanned, a plurality of scanning lines are scanned between the first and third scanning lines and between the second and third scanning lines.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/298,590, filed Dec. 12, 2005 now U.S. Pat. No. 7,187,345, whichclaims priority from Japanese patent application JP 2005-221185, filedJul. 29, 2005 and JP 2005-338009, filed Nov. 24, 2005, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an image forming method using a chargedparticle beam, and a charged particle beam apparatus. In particular, thepresent invention relates to an image forming method and a chargedparticle beam apparatus capable of suppressing the influence ofcharging.

If an electron beam is applied to a sample, secondary electrons aregenerated. In scanning electron microscopes, an observed image on thesurface of the sample is obtained using a phenomenon that the quantityof generation of secondary electrons changes depending upon the shape ofthe sample. In conventional scanning electron microscopes, scanning isconducted in the horizontal direction, i.e., in the raster direction inthe screen every line (every horizontal line in the screen). The orderof scanning lines is a descending order from the top to the bottom inthe vertical direction in the screen.

In the scan scheme according to the conventional technique, rasterscanning is conducted successively from the top to the bottom in thevertical direction in the screen. In the vertical direction, therefore,inclination is often generated in the charging phenomenon generated byelectron beam radiation.

In other words, when a certain line is being scanned, charge remainingon a line already scanned immediately before affects the primaryelectron beam and the secondary electron beam in scanning, changes theirtrajectories, and distorts a finally obtained sample image.

In JP-A-2005-142038, it is described to reduce the influence of chargingby conducting interlaced scanning using the charged particle beam.

SUMMARY OF THE INVENTION

According to the technique disclosed in JP-A-2005-142038, accumulationof charging can be mitigated to some degree by interlaced scanning.Since the time interval for forming neighboring scanning lines is notsufficient, however, there is a problem that the inclination of chargingremains.

An object of the present invention is to provide an image forming methodand a charged particle beam apparatus suitable for suppressing theinclination of charging when scanning a two-dimensional area with acharged particle beam.

According to a method, and apparatus, proposed by the present invention,a third scanning line located between a first scanning line and a secondscanning line is scanned. After the first, second and third scanninglines have been scanned, a plurality of scanning lines are scannedbetween the first and third scanning lines and between the second andthird scanning lines.

According to such a configuration, the first, second and third scanninglines have intervals including a plurality of scanning lines. Therefore,it becomes possible to prevent residual charging on one scanning linefrom affecting other scanning lines. In addition, while scanning aplurality of scanning lines between the first and third scanning linesand between the second and third scanning lines, it is possible tomitigate the charging. Over the scanning area, therefore, it is possibleto mitigate the influence of the absolute charging and suppress theinclination thereof.

As an example of such a configuration, it is desirable to locate afourth scanning line to be scanned after the scanning of the first,second and third scanning lines is located on a center line between thefirst and third scanning lines (positions at equal distances from thefirst and third scanning lines) or on a center line between the secondand third scanning lines.

According to such a configuration, the fourth scanning line is locatedin a position in the scanning area that is least susceptible to theinfluence of the charging on the first, second and third scanning lines.Therefore, it becomes possible to effectively suppress the inclinationof the charging.

Other configurations and specific examples of the present invention willbe described in detail with reference to embodiments.

According to the present invention, it becomes possible to provide amethod, and apparatus, for forming an image free from brightnessinclination.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a scanning electron microscope;

FIG. 2 is a diagram showing an example of an image memory in a scanningelectron microscope;

FIG. 3 is a diagram showing an example of a deflection pattern;

FIG. 4 is a diagram showing an example of a deflection pattern;

FIG. 5 is a diagram showing an example of a trajectory of a deflectionpattern;

FIG. 6 is a diagram showing an observation example of a contact hole;

FIG. 7 is a diagram showing an example of a line profile obtained byconducting scanning on a contact hole;

FIG. 8 is a diagram showing an example of a deflection pattern;

FIG. 9 is a diagram showing an example of a trajectory of a deflectionpattern;

FIG. 10 is a diagram showing an example of a deflection pattern;

FIG. 11 is a diagram showing an example of a trajectory of a deflectionpattern;

FIG. 12 is a diagram showing an example of a trajectory of a deflectionpattern;

FIG. 13 is a diagram showing an example of a trajectory of a deflectionpattern;

FIG. 14 is a diagram showing an example of a trajectory of a deflectionpattern;

FIG. 15 is a diagram showing an example of an image formed using ascanning method;

FIGS. 16A, 16B and 16C are diagrams showing examples in which length canbe measured with high precision irrespective of the scanning direction;

FIGS. 17A and 17B are diagrams showing examples of fast scanning;

FIG. 18 is a schematic diagram showing a preliminary charging technique;

FIG. 19 is a diagram showing brightness changes in the Y scanningdirection obtained under different scanning methods;

FIG. 20 is a diagram showing scanning in the X direction conductedalternately in opposite directions;

FIG. 21 is a diagram showing scanning of a two-dimensional area with apulse beam;

FIG. 22 is a diagram showing an example of a GUI screen for setting ascanning method; and

FIGS. 23A and 23B are diagrams showing examples of slow scanning.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In embodiments described hereafter, the order of conducting rasterscanning in the vertical direction in the screen is determined so as toalways conducting scanning between two arbitrary lines scannedpreviously in order to prevent the sample image from being distorted byinfluence of local surface charging.

By doing so, it is possible to obtain a favorable image from a samplemade of materials of two kinds described below. In other words, in thecase of a material that leaves charging on already scanned lines for along time, influences of previous scans are always balanced on a linethat is being scanned, in the vertical direction and mitigated. This isbecause influences of charging on two previously scanned lines arecanceled each other on a line located at equal distances from the lines.In the case of the other material, i.e., a material that causes chargingon an already scanned line to disappear after a definite time, alwaystaking a middle line between two previously scanned lines means alwaystaking a statistical distant place that is least susceptible to any of aplurality of previously scanned in the past, at a definite time.Finally, it becomes possible to keep the distortion of the sample imagecaused by the influence of surface charging at a minimum.

First Embodiment

Hereafter, a first embodiment of the present invention will be describedwith reference to the drawings. A first embodiment of the presentinvention is shown in FIG. 1. The present embodiment will now bedescribed by taking a scanning electron microscope which conductsscanning with an electron beam and forms a two-dimensional image of thesample as an example. However, the present embodiment is not restrictedto this. If the influence of charging cannot be neglected, it is alsopossible to apply the present embodiment to an FIB (Focused Ion Beam)apparatus, which conducts scanning with an ion beam and forms an SIM(Scanning Ion Microscope) image.

As shown in FIG. 1, the present embodiment includes an electron beamsource 1, a first converging lens 3 and a second converging lens 4 forfocusing a primary electron beam 2 emitted from the electron beam source1, deflectors 5 for deflecting the primary electron beam 2 to scan asurface of a sample 7 with the primary electron beam 2, an object lens 6for focusing the primary electron beam 2 onto a surface of the sample 7,a secondary electron detector 12 for detecting secondary electrons 16generated after the primary electron beam 2 strikes against the surfaceof the sample 7, a first converging lens power supply 8 and a secondconverging lens power supply 9 respectively for driving the firstconverging lens 3 and the second converging lens 4, a deflection signalgenerator 11 for generating deflection signals so as to scan the surfaceof the sample 7 with the primary electron beam 2 in accordance with apredetermined method, deflector drivers 10 for driving the deflectors 5in response to the deflection signal, an amplifier 13 for amplifying asecondary electron signal detected by the secondary electron detector12, an image construction apparatus for generating an image on the basisof the amplified secondary electron signal, an object lens power supply14 for driving the object lens 6 so as to focus the primary electronbeam 2 in a predetermined position, and a controller 15 for controllingthe components heretofore described.

If a line 17 a on the sample 7 is first scanned with the primaryelectron beam 2, then a line 17 b is scanned. Subsequently, a line 17 cthat is located at equal distances from the already scanned lines 17 aand 17 c is scanned.

Detailed operation of the deflection signal generator 11 and the imageconstruction apparatus 18 will now be described with reference to FIG.2. In the deflection signal generator 11, an address generation circuit22 outputs an address indicating a position on the sample 7 to which theelectron beam should be applied, in accordance with a clock output froma write clock output circuit 21. On the basis of the address, D/Aconverters 23 generate analog signals respectively corresponding toquantities by which the primary electron beam 2 should be deflectedrespectively in the horizontal direction and the vertical direction. Inaccordance with the analog signals, the deflector drivers 10 drive thedeflectors 5.

The image construction apparatus 18 operates as described below. Inother words, a secondary electron signal detected by the secondaryelectron detector 12 is amplified by the amplifier 13, and thenconverted to a digital signal by an A/D converter 24. This digitalsignal is stored in a memory group included in an illustrated imagememory 26 via an input switch 25. The selected memory group isassociated in one to one correspondence with a line indicated by anaddress that is generated by the address generation circuit 22. Thedeflection position of the primary electron beam generated by theaddress generation circuit 22 is controlled in accordance with adeflection pattern A shown in FIG. 3. Image data of lines sent from theimage memory are not in the descending order in the vertical direction(perpendicular to the lines) in the observation area on the sample. Theinput switch 25 rearranges these image data in accordance with thedeflection pattern A so as to arrange them in the descending order inthe vertical direction in the image memory.

If images corresponding to a predetermined area on the sample 7 areobtained by repeating such a procedure, the image data stored in theimage memory 26 is displayed in accordance with a procedure hereafterdescribed. That is, a read address generation circuit 28 outputs anaddress, which indicates a position on a display device 33 on whichdrawing should be conducted, in accordance with a clock output from aread clock output circuit 27. On the basis of the address, D/Aconverters 29 generate analog signals corresponding to quantities bywhich the drawing electron beam generated in the display device 33should be deflected respectively in the horizontal direction and thevertical direction. Deflection amplifiers 30 drives drive the deflectorswithin the display device 33 in accordance with the analog signals.

At this time, the image construction apparatus 18 operates as hereafterdescribed. That is, in a state in which image data are already stored inthe image memory 26, image data corresponding to one line is read outfrom a memory group in the image memory 26 indicated by an output switch31 as a digital signal. The memory group selected here is associated inone-to-one correspondence with the line indicated by the addressgenerated by the address generation circuit 28. A D/A converter 32converts the digital signal read out to an analog signal, and suppliesthe analog signal to the display device 33. In the display device 33,luminance of the drawing electron beam generated from a cathode ischanged in accordance with the analog signal. In addition, the drawingelectron beam is deflected by the deflectors included in the displaydevice 33 described earlier. As a result, an image is displayed.

The position of drawing on the display device 33 which is generated bythe read address generation circuit 28 is controlled in accordance witha deflection pattern B shown in FIG. 4. Therefore, image data of linesoutput from the image memory have a descending order in the verticaldirection (perpendicular to the lines) in the observation area on thesample. The output switch 31 sends these image data in order inaccordance with the same deflection pattern B. A scanning electronmicroscope image of the sample is displayed via these processes.

Apart from FIG. 2, the image data stored in the image memory 26 are sentto the controller 15 and subject to appropriate processing to achievethe predetermined object.

The deflection pattern A will now be described with reference to FIG. 3.In the scanning electron microscope according to the present invention,scanning is conducted in the horizontal X direction at definite periodsin accordance with a sawtooth waveform. This is called raster scan. Onthe other hand, in the vertical Y direction, the scanning position iscontrolled in conformity with the periods of the raster scan. In thecase of the deflection pattern A, a line located in a scanning lineposition “1” is scanned in a period α). A line located in a scanningline position “17” is scanned in a period β), and a line located in ascanning line position “9” is scanned in a period y). A line located ina scanning line position “13” is scanned in a period d), and a linelocated in a scanning line position “5” is scanned in a period ε).

The deflection pattern B will now be described with reference to FIG. 4.In accordance with the deflection pattern B, the deflection of theprimary electron beam is conducted in the same way as the conventionalscanning electron microscope. Each time the period advances asrepresented by a)→b)→c)→d)→e), scanning is conducted on a line locatedin a position obtained by causing a stepwise movement with apredetermined interval corresponding to one line or two lines.

FIG. 5 shows an example of a trajectory obtained when the sample 7 isscanned with the primary electron line 2 in accordance with thedeflection pattern A.

For example, in an arbitrary area formed by seventeen lines as shown inFIG. 5, an Nth scan, an (N+1)-st scan, an (N+2)-nd scan, an (N+3)-rdscan and an (N+4)-th scan are conducted on a line 1, a line 17, a line9, a line 13 and a line 5, respectively. In other words, the (N+3)-rdscan is conducted on a center line in an area (having a first scanningline interval) prescribed by lines respectively associated with the(N+1)-st scan and the (N+2)-nd scan, and the (N+4)th scan is conductedon a center line in an area (having a first scanning line interval)prescribed by lines respectively associated with the Nth scan and the(N+2)-nd scan.

In intervals (having a second scanning line interval) between two of thelines “1,” “5,” “9,” “13” and “17,” a plurality of scanning lines stillremain to be scanned. Therefore, it becomes possible to prevent thecharging generated on each scanning line from affecting other scanninglines. Subsequently, any of lines “3,” “7,” “11” and “15” is scanned.This scan on the lines “3,” “7,” “11” and “15” corresponds to thefollowing operation: after the scan on center lines between scanninglines having equal scanning line intervals (scans on the lines “5” and“13”) has been finished, scan is newly conducted on scan lines havingnarrowed intervals from the already scanned scan lines (the lines “1,”“5,” “9,” “13” and “17”). By thus repeating a process of subsequentlyconducing scanning on a scanning line located on a center line betweenalready scanned lines, it becomes possible to suppress the inclinationof charging. This effect becomes more remarkable when the number ofscanning lines is large. When scanning M scanning lines, a line 1 isfirst scanned and a line M is secondly scanned, and then a line mlocated on a center line between the line 1 and the line M is thirdlyscanned. (If M is even, a scanning line that is the closest to thecenter line between the line 1 and the line M is selected.)

Subsequently, the line 1 and the line m, or the line m and the line Mare regarded as the original line 1 and line M, and a center linebetween them is scanned. By repeating such processing and conductingprocessing of gradually narrowing the interval between the line 1 andthe line M, it is possible to sustain the effect of preventing thecharging phenomenon of each scanning line from affecting other scanninglines for a long time.

Owing to the configuration heretofore described, it is possible toimplement a scanning electron microscope capable of suppressing theinclination of the charging phenomenon caused in the directionperpendicular to the raster scan by scanning on the sample with theelectron beam. In the case of the present example, a new scanning lineis set in a position that is at the greatest distance from the alreadyscanned position, and an interval between consecutive scanning lines ismade equal to an interval corresponding to a plurality of scanninglines. As a result, it becomes possible to suppress the rise of thecharging value over the entire scanning area, and it becomes possible toeffectively prevent the inclination of the charging phenomenon. Whenscanning one frame or one area, finally a new scanning line is set in aposition that is in close vicinity to an already scanned position. Atthat time, however, the charging state on the already scanned scanningline is mitigated to some degree. As a whole, therefore, it becomespossible to suppress the charging state.

An example of an effect obtained using the scanning electron microscopeaccording to the present embodiment is shown in FIG. 6. If a hole formedby a specific material and called contact hole is observed usingconventional TV scanning, an ellipse that is longer in the verticaldirection is obtained as represented by a dot-dash line. By using thescanning method according to the present invention, however, it becomespossible to observe the contact hole in its original shape that is closeto the real circle and shown by a solid line. A profile of the samecontact hole image along a section A-B is shown in FIG. 7. In the imageobtained using the conventional TV scanning and represented by thedot-dash line in FIG. 6, the profile is shown to be higher on a B sidelocated on the bottom side in the vertical direction on the screen andscanned last as compared with an A side located on the top side andscanned first. This indicates that the image is displayed brightly onthe B side as compared with the A side. In an image obtained by the scanmethod according to the present invention and represented by a solidline in FIG. 6, however, there is no inclination, i.e., there is nobrightness difference on the profile between the A side and the B side.

A deflection pattern C used when linking predetermined areascontinuously to form one screen will now be described with reference toFIG. 8. FIG. 8 shows an example obtained when two areas are linked toform one screen.

In the present example, “a” and “e” lines located respectively in areas1 and 2 are first scanned in order. In the present example, a1 isscanned in a period a1, and a2 is scanned in a period b1. Then e1 isscanned in a period c1, and e2 is scanned in a period d1.

Subsequently, two “g” lines are scanned extending over areas. And two“c” lines located between the already scanned “a” and “e” lines arescanned. And remaining scanning lines are scanned, and all scanninglines are scanned.

It becomes possible to mitigate the influence of the charging by settinga position that is located at the greatest distance from a plurality ofscanning lines as the next scanning position on the basis of thepreviously scanned scanning position. Finally, two areas taken in arelinked to form one screen.

FIG. 9 shows an example of a trajectory obtained by scanning the sample7 with the primary electron beam 2 according to the above-describeddeflection pattern C.

In the example shown in FIG. 9 as well, it is desirable to conduct theprocessing of repeating the process in which a center line betweenalready scanned lines is scanned as the next scanning line as describedwith reference to the example shown in FIG. 5. In the present example,the example in which a scanning area is divided into two parts andprocessed has been described. However, this is not restrictive. Forexample, the area may be divided into four parts, eight parts, sixteenparts, thirty-two parts, or sixty-four parts. If the number of partsobtained by the division is too large, the effect of the presentembodiment is decreased. Therefore, it is desirable to limit the numberof parts to approximately sixty-four. The reason will now be described.If the number of scanning lines is too large, the next scanning line isscanned in the adjacent position in the state in which the influence ofcharging generated by the previous scanning line remains, resulting ininclination of charging. If scanning is conducted so as to produce acommon scanning sequence every area produced by the division, it ispossible to prevent the charging state from inclining every area. Inaddition, it is possible to provide the controller with a function ofchanging over the number of parts obtained by the division and changeover the scanning pattern according to the changeover in the number ofparts.

FIG. 10 shows an example obtained when the present invention is appliedto the interlaced scanning. In the interlaced scanning, lines areselected alternately and scanned, and remaining lines are scanned at thenext step. By the way, in the conventional TV scanning, the interlacedscanning is basically adopted. FIG. 10 shows the case where two areasconstruct a screen. In the present example, first, all lines of a_evenlocated first in areas 1 to 2 are scanned in order. In FIG. 10, a1_evenis scanned in a period a1 and a2_even is scanned in a period b1. Thenc1_even is scanned in a period c1 and c2_even is scanned in a period d1.

Subsequently, extending over areas, c_even lines located between twoalready scanned a_even lines are scanned in the areas 1 to 2. In FIG.10, c1_even is scanned in a period y1 and c2_even is scanned in a periodδ1. Then d1_even is scanned in a period α2 and d2_even is scanned in aperiod β2.

Subsequently, d_even lines located between the already scanned c_evenlines and a_even lines in the next area, and b_even lines locatedbetween the a_even lines and c_even lines are scanned in the same way.To this point, scanning corresponding to one field “even” in theinterlaced scanning is finished.

Lines corresponding to “odd” located between lines alternately scannedin “even” are scanned one after another in periods α3 to δ4 in the orderof a_odd→c_odd→d_odd→b_odd in the same way as “even.” The remaining onefield corresponding to “odd” is taken in. Finally, one field in the“even” and one field in the “odd” are compounded to construct onescreen.

FIG. 11 shows an example of a trajectory obtained when the sample 7 isscanned with the primary electron line 2 in accordance with a deflectionpattern D. For example, it is now assumed that one screen is formed oftwo areas as shown in FIG. 11. An Nth scan and an (N+1)-st scan areconducted respectively on a line a1_even and a line a2_even in the nextarea. Returning to the previous area, an (N+2)-nd scan is conducted on aline c1_even. An (N+3)-rd scan is conducted on a line c2_even in thenext area again. Hereafter, scans are conducted on lines correspondingto “even” in the interlaced scanning until an (N+7)-th scan on a lineb2_even. After the scans corresponding to “even” lines in the interlacedscanning have been finished, (N+8)-th to (N+15)-th scans are conductedon lines corresponding to “odd” in the same way.

FIG. 12 shows an example in which only one line is exempted from therule of the present embodiment and a scan is conducted on a screen endwithout taking a center line between two lines scanned earlier. Even ifsuch scanning is conducted, there is no substantial change in the effectof the present invention. In other words, even if scans overlap at thescreen end and partial inclination of charging occurs in the portion,the charging situation in a principal portion of the screen is notaffected. Furthermore, even if two lines are added at the screen endinstead of only one line as shown in FIG. 13, there is no change in theeffect of the present invention. Furthermore, even if only one line isscanned not at the bottom end of the screen but at the top end as shownin FIG. 14 without taking a center line between two lines scannedearlier, there is no change in the effect of the present invention.

Second Embodiment

An example in which pattern length measurement is conducted on the basisof an image formed using a scanning method of scanning a center linebetween scanning lines one after another as described above will now bedescribed. FIG. 15 is a diagram showing an example of an image formedusing the scanning method of the present example.

On this image, a line pattern bent in a part thereof by 90 degrees isdisplayed. When measuring the line width of such a line pattern, thescanning line direction of the raster scan and the sample direction areset in the conventional technique so as to make the pattern edgeperpendicular to the scanning line direction of the raster scan. Becausea length value measured in the scanning line direction of the rasterscan becomes different from that measured in other directions asillustrated in FIG. 16 although the line width is the same. Since inthis way the length value measured in the scanning line directionbecomes different from that measured in other directions, there is aproblem that the length value measured in a direction other than thescanning line direction cannot be ensured.

As cause of difference in measured length value or magnification betweenthe scanning line direction (such as the X direction) and anotherdirection (such as the Y direction), influence of charging isconceivable. If one scanning line is scanned and then scanning isconducted in an adjacent place, scanning using the electron beam isconducted at an interval different from the intended scanning lineinterval under the influence of charging in the previously scannedplace. Therefore, it is considered that the magnification in the Xdirection and that in the Y direction do not have the relation of 1:1.

Such a problem is solved in the present example as follows. Whenscanning a two-dimensional area on the sample by using a plurality ofscanning lines, a third scanning line is scanned between a firstscanning line and a second scanning line, and then a plurality ofscanning lines are scanned between the first scanning line and thesecond scanning line and between the second scanning line and the thirdscanning line. In addition, the length measurement direction can be setin a direction different from a scanning line direction of an imageformed on the basis of the scanning.

Specifically, a length measurement range setting box 40 is provided asshown in FIG. 15 so as to be capable of setting the length measurementdirection in a direction (which is a direction perpendicular to thescanning line direction in the case of the present example) differentfrom the scanning line direction. Owing to the scanning specific to thepresent example, it is possible to suppress the magnification errorcaused between the scanning line direction and another direction(especially, a direction perpendicular to the scanning line direction).According to the present example, therefore, it becomes possible tomeasure the length accurately irrespective of the direction. This effectis demonstrated especially when measuring the length as shown in, forexample, FIG. 15.

In the example shown in FIG. 15, two length measurement points 41 and 42that are different in length measurement direction are present on oneimage. When conducting such length measurement, it is necessary in theconventional technique to change the scanning line direction every twolength measurement points in order to ensure the length measurementprecision. The present example has an effect that length measurement ina plurality of directions can be conducted at high precision on onescreen. If the actual illustrated line pattern has the same line width,it is possible in the present example to square a line width L in the Xdirection with a line width Lrr in the Y direction with high precisionirrespective of the scanning direction.

In addition, conducting the above-described scanning also brings aboutan effect that the length can be measured with high precision even ifthe scanning line direction is rotated and the scanning line directionis set to an arbitrary direction. It also becomes possible to maintainhigh length measurement precision irrespective of the scanning directionby rotating the scanning direction (i.e., conducting raster rotation) asdescribed in FIG. 16.

By the way, the length measurement range setting box 40 can be displayedon the formed image, and positions of measurement reference positions 43and 44 can be arbitrarily set. The measurement reference positions 43and 44 can be set in arbitrary positions on the image by using, forexample, a pointing device, which is not illustrated. The controller 15is programmed so as to, for example, store the length measurement rangesetting box 40, count pixels between the measurement reference positions43 and 44 in the length measurement range setting box 40, and measurethe length between the measurement reference positions 43 and 44 on thebasis of magnification set at that time.

Third Embodiment

An example of further mitigating the influence of the charging bycombining a faster scanning rate with the scanning method described withreference to the first embodiment will now be described. FIGS. 17A and17B are diagrams showing deflection patterns (deflection patterns E andF represented by solid lines) used when scanning using the electron beamis conducted at a scanning rate that is twice with respect to thedeflection patterns A and B described in the first embodiment.Observation and measurement become possible even for a sample affectedby the charging more remarkably by combining the suppression of chargingobtained using scanning faster than the ordinary scanning with thescanning method described with reference to the first embodiment.

FIG. 22 is a diagram showing an example of a GUI (Graphical UserInterface) for setting various scanning methods including the scanningmethod of the present example and the scanning rate. In a GUI screenshown in FIG. 22, a window for selecting an ordinary television scan orthe scanning method described in the first embodiment (represented as“anti-charge” scan because it is a scanning method that is effectiveagainst charging) and a window for selecting a scanning rate (fastscanning, normal scanning, and slow scanning) are included. Here, thefast scanning refers to scanning conducted at the scanning rate as shownin FIGS. 17A and 17B (supposing that scanning conducted at the scanningrate as shown in FIGS. 3 and 4 is normal scanning). Scanning conductedat a scanning rate as shown in FIGS. 23A and 23B is slow scanning.

If the TV scan is selected, inhibition processing is conducted in thepresent example to prevent the slow scan from being selected. If theanti-charge scan is selected, it is allowed to select the slow scan.Since the scanning method described in the first embodiment is notsusceptible to the influence of charging, scanning can be conducted at arelatively slow rate as compared with the TV scan.

Therefore, the slow scan is made selectable when the scanning methoddescribed in the first embodiment is selected, whereas the slow scan isnot made selectable when the scanning method described in the firstembodiment is not selected. When scanning a sample that is easilyaffected by the charging, this prevents a combination of the scanningmethod that stores charging on the sample (TV scan) with the slow scanthat stores charging on the sample (a deflection pattern H) from beingfalsely selected.

If the TV scan is selected and then the fast scan, the normal scan orthe slow scan is selected, the deflection pattern F, the deflectionpattern B or the deflection pattern H is selected in the presentexample, respectively. If the anti-charge scan is selected and then thefast scan, the normal scan or the slow scan is selected, then thedeflection pattern E, the deflection pattern A or the deflection patternG is selected, respectively.

As heretofore described, the influence of charging on the sample differsdepending upon the scanning method with the electron beam. By changingthe setting range of selectable parameters concerning the electron beamaccording to the scanning method, therefore, it becomes possible to setarbitrary parameters without considering the influence of charging. Inthe present example, the setting parameters are the scanning method andthe scanning rate. However, this is not restrictive. It is also possibleto set the kind of the sample that differs in influence of charging as apart of the setting parameters. At this time, it is conceivable toconduct inhibition processing, such as preventing the slow scan frombeing selected or preventing the raster scan from being selected, when asample kind that is easily affected by the charging is selected.Furthermore, the beam current and magnification can become a part of thesetting parameters. In the present example, desired setting can beconducted using a pointing device which is not illustrated. Theparameters set on the GUI screen shown in FIG. 22 are sent to thecontroller 15, and converted to control signals for the deflectors.

Fourth Embodiment

In the pre-charging technique, the sample surface is charged previouslyby applying an electron beam, and scanning is conducted with an electronbeam for forming an image under the charged state. An example in whichthe above-described scanning method is applied to the pre-chargingtechnique will now be described.

FIG. 18 is a schematic diagram showing the pre-charging technique(pre-dose). The pre-charging technique is a technique of charging thesample surface positively by electron beam irradiation in order to pullup electrons from, for example, the bottom of a contact hole of asemiconductor device. It becomes possible to pull up secondary electronsefficiently from the bottom of the contact hole by applying the electronbeam for forming an image under the positive charging state. For thatpurpose, scanning with an electron beam for previously charging thesample surface is conducted before scanning with an electron beam forforming an image is executed. Such a pre-charging technique is describedin JP-A-5-151927 (corresponding to U.S. Pat. No. 5,412,209) andJP-A-2000-200579 (corresponding to U.S. Pat. No. 6,635,873).

In this example, it is proposed to use a scanning method of scanning athird scanning line between a first scanning line and a second scanningline, and then scanning a plurality of scanning lines between the firstscanning line and the second scanning line and between the secondscanning line and the third scanning line, as the scanning for thepre-charging. The pre-charging scanning aims at charging the samplesurface. The prime object of the pre-charging scanning is to detectsecondary electrons from the bottom of the stable contact hole with highefficiency by sustaining the charged state. If charging is conductedwith inclination, therefore, a bad influence is exerted on the scanningfor forming an image over a long time.

In view of such circumstances, in the present example, the pre-chargingscanning as described above is conducted and then the scanning forforming the image is conducted. By thus conducting the scanning, itbecomes possible to form a stable charging state having no inclination.In the example described with reference to FIG. 18, magnification in thepre-charging scanning is made lower than that for forming the image.However, this is not restrictive. It is also possible to conduct thepre-charging scanning by scanning the sample surface with an electronbeam having arrival energy with a secondary electron emission efficiencyδ>1.

FIG. 19 is a diagram showing a change of brightness in the Y scanningdirection obtained when the pre-charging scanning is conducted using theordinary TV scan and when the pre-charging scanning is conducted usingthe scanning method in the present example. A dot-dash line represents achange of image brightness obtained when the TV scan is conducted. Inthe ordinary scanning, the next scanning line is scanned in the vicinityof a scanning line, and this is repeated. As the position moves from thetop of the image to the bottom, therefore, charging is graduallyaccumulated. Since the top of the image is thus different in chargingstate from the bottom, the top differs in brightness from the bottom.

If scanning for measurement is conducted in such a state, a line profilerepresenting a change of the detected electron quantity and pixelbrightness becomes as represented by the dot-dash line in FIG. 19 underthe influence of inclined charging formed at the time of pre-charging.On the other hand, if the pre-charging is conducted using the scanningmethod of the present example, the charging state is stable irrespectiveof the position in the Y direction. Therefore, it becomes possible toform a proper line profile that represents the shape of the samplesurface.

Fifth Embodiment

FIG. 20 shows a scanning method obtained by further improving thescanning method described in the first embodiment. Scanning in the Xdirection is conducted alternately in opposite directions. That is, theline 1 is first scanned from the left side in FIG. 20 to the right side.Subsequently, the line 17 is scanned from the right side in FIG. 20 tothe left side. Even if the sample is a sample on which charging isinclined depending upon the scanning direction and the influence ofcharging is accumulated, it becomes possible to form a sample imagehaving high horizontal uniformity irrespective of the influence ofcharging, by thus changing the scanning direction every scanning line.

Sixth embodiment

FIG. 21 is a diagram showing an example in which the scanning method ofscanning the next scanning line located on a center line between alreadyscanned scanning line trajectories described in the first embodiment isdeveloped two-dimensionally. In the case of the present example, first,an electron beam (N) (pulse beam) is applied to coordinates (17, 1) on acorner of a two-dimensional irradiation area in a spot form.Subsequently, an electron beam (N+1) is applied to coordinates (1, 17),which is at the longest distance from the coordinates (17, 1) in thescanning subject area. Subsequently, electron beams (N+3 and N+2) areapplied to coordinates (1, 1) and coordinates (17, 17), which arelocated between the coordinates (1, 17) and the coordinates (17, 1) andwhich are at the longest distance from the coordinates (1, 1) andcoordinates (17, 17). Thereafter, an electron beam (N+4) is applied tothe center (9, 9) among irradiation positions to which the electronbeams have been applied. In addition, electron beams (N+5, N+6, N+7, andN+8) are applied to centers among irradiation positions to which theelectron beams have been applied. In addition, thereafter, applyingelectron beams to centers among irradiation positions to which theelectron beams have been applied is repeated. As a result, irradiationover the entire scanning area is completed.

By the way, the above-described scanning sequence is nothing but anexample. For example, after the electron beam (N+4) has been applied,the next irradiation point may be set to a center of each of sidesprescribing the two-dimensional irradiation area, such as coordinates(9, 17) between the coordinates (1, 17) and the coordinates (17, 17). Itis desirable to position the next irradiation point on a center betweentwo previous irradiation points and set the next irradiation point in aposition that is at the longest distance from a point of lastirradiation and a point of last irradiation but one, or a position closeto the position. After beam irradiation to center positions having equalintervals between irradiation positions on coordinates has beenfinished, preferably the next irradiation positions are set to centerpositions having narrower intervals between irradiation positions, andthe intervals between irradiation positions are gradually narrowed.

By repeating deflections that position the next irradiation positions onthe centers among already irradiated irradiation positions, irradiationover all coordinates spreading two-dimensionally is completed.

By the way, even if an electron beam is applied in a pulse form asheretofore described, an image of a two-dimensional area on the samplecan be formed by synchronizing the signal supplied to the scanningdeflector with the deflection signals of the display device in the sameway as the ordinary scanning electron microscope. When moving the beambetween irradiation points, it becomes possible to selectively apply theelectron beam to a desired irradiation position by diverting thetrajectory of the electron beam from the sample with, for example, adeflector for blanking so as to prevent the electron beam from beingapplied to the sample.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An image forming method for scanning a two-dimensional area on asample with a charged particle beam and forming an image of the scannedarea, the image forming method comprising the steps of: scanning a firstscanning line; scanning a second scanning line; scanning a thirdscanning line between the first scanning line and the second scanningline; scanning an area having two sections, defined by the first,second, and third scanning lines, by sequentially scanning positionsbetween the first and third scanning lines and scanning positionsbetween the third and second scanning lines so as to reduce the area instages; and repeatedly scanning for a center by scanning lines betweenthe first scanning line and the third scanning line, and scanning linesbetween the third scanning line and the second scanning line untilscanning of the two-dimensional area is complete.
 2. An image formingmethod according to claim 1, wherein said third scanning line isdisposed at equidistance from said first and second scanning lines. 3.An image forming method according to claim 1, wherein scanning linedisposed at the center of said plurality of scanning lines is a fourthscanning line which is scanned next to said third scanning line.
 4. Animage forming method according to claim 1, wherein said first and secondscanning lines are disposed at the edge of said two-dimensional area ora part of area of said two-dimensional area.
 5. A charged particle beamapparatus including a deflector for scanning charged particle beam on asample and a control apparatus for controlling said deflector, wherein:said control apparatus controls said deflector to: scan a first scanningline and second scanning line on a two-dimensional area on said sample;scan a third scanning line between said first and second scanning lines;scanning an area having two sections, defined by the first second, andthird scanning lines, by sequentially scanning positions between thefirst and third scanning lines and scanning positions between the thirdand second scanning lines so as to reduce the area in stages; andrepeatedly scanning for a center by scanning lines between the firstscanning line and the third scanning line, and scanning lines betweenthe third scanning line and the second scanning line until scanning ofthe two-dimensional area is complete.
 6. A charged particle beamscanning apparatus according to claim 5, wherein: said control apparatuscontrols said deflector such that positioning said third scanning lineat equidistance to said first and second scanning lines.
 7. A chargedparticle beam apparatus according to claim 5, wherein: said controlapparatus renders scanning line disposed at center of said plurality ofscanning lines as a fourth scanning line which is scanned next to saidthird scanning line.
 8. A charged particle beam apparatus according toclaim 5, wherein: said control apparatus positions said first and secondscanning lines at an edge of said two-dimensional are or part of area ofsaid two-dimensional area.
 9. A charged particle beam apparatusaccording to claim 5, wherein: said control apparatus controls saiddeflector such that structuring said two-dimensional are on said sampleas a single or divided plural areas, and scanning said first, second andthird scanning lines at each area.
 10. A charged particle beam apparatusaccording to claim 9, wherein: said two-dimensional area is divided intoany one of two parts, four parts, eight parts, sixteen parts, thirty-twoparts and sixty-four parts.
 11. An image forming method for scanning atwo-dimensional area on a sample with a charged particle beam andforming an image of scanning area, the image forming method comprisingthe steps of: scanning a first scanning line at upper portion of saidtwo-dimensional area on said sample, a second scanning line at lowerpart, and third scanning line between said first and second scanninglines and at the center of said two-dimensional area; and then, furtherscans scanning lines between said first and third scanning lines andbetween said second and third scanning lines, wherein said furtherscanning of said scanning lines repeat scanning of new scanning lines ata center of scanning lines which already scanned, such that narrowingdistance between said scanning lines already scanned and scanning linesnewly scanned, and wherein scanning is repeatedly performed for a centerby scanning between the first scanning line and the third scanning line,and scanning lines between the third scanning line and the secondscanning line until scanning of the two-dimensional area is complete.12. An image forming method according to claim 11, wherein: saidscanning of said new scanning lines scans scanning line newly fornarrowing distance of scanning lines which is already scanned, afterscanning at the center between scanning lines haying a same scanningline distance is said two-dimensional area.
 13. An image forming methodaccording to claim 11, wherein: said second scanning line scans at farmost position from said first scanning line in said two-dimensionalarea.
 14. A charged particle beam apparatus including deflector forscanning charged particle beam on a specimen and a control apparatus forcontrolling said deflector, wherein: said control apparatus scans afirst scanning line at an upper portion of two-dimensional area on saidspecimen scans, a second scanning line at a lower part of saidtwo-dimensional area, scans a third scanning line at the center of saidtwo-dimensional area, and scans scanning lines between said first andsaid third scanning lines, and between said second and said thirdscanning lines, wherein said further scanning of said scanning linescontrols said deflector such that repeat scanning of new scanning linesat a center of scanning lines which already scanned, and narrowingdistance between said scanning lines already scanned and scanning linesnewly scanned, and wherein scanning is repeatedly performed for a centerby scanning between the first scanning line and the third scanning line,and scanning lines between the third scanning line and the secondscanning line until scanning of the two-dimensional area is complete.15. A charged particle beam apparatus according to claim 14, wherein:said control apparatus controls said deflector such that scans said newscanning lines for narrowing distance of scanning lines already scanned,after scanning at the center of scanning lines having a same scanningline distance in said two-dimensional area.
 16. A charged particle beamapparatus according to claim 14, wherein: said control apparatuscontrols said deflector such that said second scanning line is scannedat far most position from said first scanning line in saidtwo-dimensional area.
 17. A pattern dimension measurement method forscanning two-dimensional area on a specimen with a charged particle beamand for forming an image of scanning area, and measuring dimension ofpattern formed on said specimen based on said image, said methodcomprising steps of: as said two-dimensional area of said specimen,scanning a first and second scanning lines and scanning a third scanningline between said first and second scanning lines, then, scanning aplurality of scanning lines between said first and third and scanninglines and between said second and third scanning lines so that formingimage of said two-dimensional area, and measuring said pattern bysetting measuring direction in a different direction of line directionof each scanning lines on said image; scanning an area having twosections, defined by the first, second, and third scanning lines, bysequentially scanning positions between the first and third scanninglines and scanning positions between the third and second scanning linesso as to reduce the area in stages; and repeatedly scanning for a centerby scanning lines between the first scanning line and the third scanningline, and scanning lines between the third scanning line and the secondscanning line until scanning of the two-dimensional area is complete.18. An image forming method for scanning a two-dimensional area on asample with a charged particle beam and forming an image of scanningarea, the image forming method comprising the steps of: as to areaincluding two-dimensional area in said sample, scanning a first andsecond scanning lines, scanning a third scanning line between said firstand second scanning lines, then, by scanning a plurality of scanninglines between said first and third scanning lines, between second andthird scanning lines, rendering to charge area including two-dimensionalarea of said sample, forming an image of said scanning area by scanningsaid charged particle beam onto said charged two-dimensional area,scanning an area haying two sections, defined by the first, second, andthird scanning lines, by sequentially scanning positions between thefirst and third scanning lines and scanning positions between the thirdand second scanning lines so as to reduce the area in stages; andrepeatedly scanning for a center by scanning lines between the firstscanning line and the third scanning line, and scanning lines betweenthe third scanning line and the second scanning line until scanning ofthe two-dimensional area is complete.
 19. A charged particle beamapparatus for deflecting charged particle beam irradiated on a specimenin a spot like shape, a deflector, and control apparatus for controllingsaid deflector, wherein: said control apparatus controls said deflectorsuch that deflects said charged particle beam so that said chargedparticle beam is irradiated at a first irradiating position positionedat four corners of said two-dimensional area on said specimen, thendeflects said charged particle beam so that a second irradiatingposition is positioned at the center of said first irradiatingpositions, then scans an area having two sections, defined by a firstscanning line, a second scanning line, and a third scanning line betweenthe first and second scanning lines, by sequentially scanning positionsbetween the first and third scanning lines and scanning positionsbetween the third and second scanning lines so as to reduce the area instages, and repeatedly scans for a center by scanning lines between thefirst scanning line and the third scanning line, and scanning linesbetween the third scanning line and the second scanning line untilscanning of the two-dimensional area is complete.